1. Field of the Invention
This invention relates to a zoom lens.
2. Related Background Art
Generally in a zoom lens, focusing is effected by that lens group thereof which is most adjacent to the object side being moved. This focusing method is still used in many zoom lenses because in a zooming range for objects on the same distance, focusing can be accomplished by a substantially constant amount of movement and moreover the structure of the lens barrel can be constructed relatively simply.
However, that lens group of a popular zoom lens which is most adjacent to the object side requires the largest lens diameter and therefore, the zoom lens becomes heavy, and this leads to the disadvantage that in case of auto focusing, quick focusing becomes difficult.
So, in recent years, the lighter weight of a lens group for focusing has been desired with the spread of auto focus cameras and various focusing systems have been proposed.
Zoom lenses disclosed, for example, in Japanese Laid-Open Patent Application No. 52-109952, Japanese Laid-Open Patent Application No. 56-21112, Japanese Laid-Open Patent Application No. 57-4018 and Japanese Laid-Open Patent Application No. 1-204013 are known.
In the zoom lens disclosed in the above-mentioned Japanese Laid-Open Patent Application No. 52-109952, focusing is effected by a part of that lens group thereof which is most adjacent to the object side being moved, and in a zooming range for objects on the same distance, focusing can be accomplished by a substantially constant amount of movement and the structure of the lens barrel can be constructed relatively simply. However, the full length of the lens group which is most adjacent to the object side is increased and a movable lens for focusing is disposed in the lens group of large lens diameter which is most adjacent to the object side, and this leads to the disadvantage that a great decrease in the weight of the focusing group cannot be very much expected. In the zoom lens of Japanese Laid-Open Patent Application No. 56-21112, focusing is effected by some of lens groups posterior of the object side lens group being moved, and in a zooming range for objects on the same distance, focusing can be accomplished by a substantially constant amount of movement and the structure of the lens barrel can be made relatively simple. However, this zoom lens is restricted to the four-group afocal zoom lens type and is not popular.
Also, in the focusing system disclosed in Japanese Laid-Open Patent Application No. 57-4018, any other lens group than that lens group which is most adjacent to the object side is used as a focusing group. Therefore, the amount of movement differs depending on each vari-focused state for objects on the same distance. Specifically, focusing is effected by the use of a zooming cam and a focusing cam which are function-approximated to each other in the correction of the amount of movement. This method has the feature that a group small in the fluctuation of a short distance or a group small in weight can be freely selected, but has the disadvantage that the structure of the lens barrel becomes complicated.
A system for an integral lens type camera is disclosed in Japanese Laid-Open Patent Application No. 1-204013. This is a system in which each vari-focused state is read by an electrical signal and an amount of movement conforming thereto is found. This system also has the feature that a group small in the fluctuation of a short distance or a group small in weight can be freely selected, but has the disadvantage that it cannot be used as a zoom lens for a single-lens reflex camera of the manual focusing type.
Further, in a lens exclusively for auto focusing or the like, when the correction of the amount of movement is effected in accordance with the focal length within the zooming range, it is preferable that the following condition be satisfied in the entire zooming range: EQU .vertline..beta..sub.F .vertline.&gt;1.8 CONDITION (2)
The above-mentioned substantially afocal system refers to a lens system in a state in which the lateral magnification .beta..sub.F of a first sub-group in a second lens group in case of an infinity state is .vertline..beta..sub.F .vertline..gtoreq.100.
When optical systems approximate to each other in small thickness are supposed, in succession from the object side, a lens group on the object side which is not concerned in focusing is defined as (A), a focusing lens group concerned in focusing is defined as (F) and on the image side, a lens group on the image side which is not concerned in focusing is defined as (B). The focal length of the lens group (A) on the object side which is not concerned in focusing is defined as f.sub.A, the focal length of the focusing lens group (F) is defined as f.sub.F, the lateral magnification of the focusing lens group (F) when the photographing distance is infinity is defined as .beta..sub.F, the photographing distance (the distance between object images) is defined as R, and the amount of movement by which the focusing lens group (F) is moved to be focused on an object at the photographing distance R is defined as .DELTA.x (the movement from the object side to the image side is positive). Also, the length of the entire lens system is defined as TL (the length from the object side principal point to the image plane of the lens group (A) on the object side).
When the distance from the object side principal point of the lens group (A) on the object side to the object is D.sub.0, the amount of movement .DELTA.x for focusing is expressed by the following equation I as described in a publication Optics, Vol. 12, No. 5, October, 1983, pp. 359-366 ("The New Focusing System for Zoom Lens"). ##EQU1##